You ask "How often should a septic tank be pumped?"
Ive heard septic suckers say one should do it like at least yearly (they just may have a vested interest ya think lol???)
Ive heard others say NOT UNLESS THEY ARE CAUSING A PROBLEM
I SAY it sorta depends on the use (how many in family),,,,,,,,how good a drain field you have and how well it perks,,,,,,,,,how much grease n soap (kitchen sink) and how much lint (clothes washer) you out in it
So I say the answer is it DEPENDS and is likely somewhere in the middle of the 2 above extremes, BUT IM NOT A PLUMBER